Machines including, for example, passenger vehicles, generators, and earth moving vehicles utilize a variety of heat exchangers during operation. Heat exchangers may be used to modify or maintain the temperature of fluids circulated throughout machines. For example, an internal combustion engine is generally fluidly connected to several different liquid-to-air and/or air-to-air heat exchangers (e.g., oil cooler, radiator, air cooler) to cool liquids and gases circulated throughout the engine. The circulated fluids may include oil, coolant, exhaust gas, air, or other fluids used in various machine operations.
In general, heat exchangers are devices that transfer thermal energy between two fluids without direct contact between the two fluids. A primary fluid is typically directed through a fluid conduit of the heat exchanger, while a secondary cooling or heating fluid is brought into external contact with the fluid conduit. In this manner, thermal energy may be transferred between the primary and secondary fluids through the walls of the fluid conduit. The ability of the heat exchanger to transfer thermal energy between the primary and secondary fluids depends on, amongst other things, the surface area available for heat transfer and the thermal properties of the heat exchanger materials.
Governments, regulatory agencies, and customers are continually urging machine manufacturers to increase fuel economy, meet lower emission regulations, and provide greater power densities. These demands often lead to increased requirements for thermal energy transfer in the machine's heat exchangers (e.g., a higher power density for a combustion engine may increase the amount of thermal energy created during the operation of the engine, which must subsequently be removed by the radiator and/or oil cooler to ensure proper operation). As a result, machine manufacturers must develop new materials and/or methods for increasing the ability of heat exchangers to transfer heat.
Metal foams have been used in heat exchangers to increase the surface area available for heat transfer. One method of using a metal foam to improve the ability of a heat exchanger to transfer heat is described in U.S. Pat. No. 7,131,288 (the '288 patent), issued to Toonen et al. on Nov. 7, 2006. In particular, the '288 patent discloses a heat exchanger that comprises a number of parallel flow passages that are arranged at a distance from one another and have an elliptical cross section, through which a first fluid, for example a liquid, is guided. A flow body comprises two metal foam parts, each with a gradient of the volume density parallel to the direction of flow of the second fluid (e.g., a gas). In the first metal foam part, the volume density (amount of metal) increases in the direction of flow of the second fluid, while in the second metal foam part the volume density decreases in the direction of flow. Consequently, most metal is present in the immediate vicinity of the flow passages, where the highest heat flux density also prevails. The outer surface of the flow body, in particular the inflow side (and discharge side), is relatively open. The heat exchanger of the '288 patent is preferably of modular structure, so that a plurality of modules can be combined to form a larger unit.
Although the heat exchanger of the '288 patent may use a metal foam to increase heat transfer, it may still be problematic. Specifically, if the heat exchanger is manufactured by forming the metal foam around the passages, the metal foam may at least partially shrink away from the passages during cooling, resulting in poor contact. This foam shrinkage may result in increased resistance to thermal energy transfer between the passage and the metal foam and, thus, reduced performance. Furthermore, due to the low volume density of metal at the outer surface of the metal foam, mechanically and thermally bonding the metal foam to other surfaces (e.g., metal plates, other modules, etc.) may be difficult.
The disclosed heat exchanger is directed to overcoming one or more of the problems set forth above.